This invention generally concerns an improved system and method for protecting television receiver circuitry from overvoltage surges caused by cathode ray tube arc-over and more specifically is directed to a cathode ray tube socket incorporating controlled, or tuned, spark gaps for dissipating to neutral ground potential cathode ray tube overvoltage surges which exceed a precisely specified voltage threshold in providing protection for television receiver circuitry.
Occasionally large transient voltages are generated within the cathode ray tube (CRT) of a television receiver. These high voltage surges in the CRT are caused by arc-over between various tube components. CRT arcing is due to voltage discharge from regions of the tube at anode potential to elements of the tube initially at lower potentials. This discharge momentarily increases the potential of these elements to values much higher than that for which they were designed. Not only can the CRT be damaged by arc-over, but damage to circuit elements in the television receiver itself may occur if the high potential appears at the connector pins of the CRT. Unless these voltages are controlled or dissipated, they may be conducted through the tube pins and CRT socket contacts to other portions of the circuitry associated with the tube. Even if such arc-over causes no CRT or television receiver damage, the arc generally produces a loud noise and disrupts the video presentation to the annoyance of the viewer.
It is generally known in the art that most CRT arc-over is caused by loose particles in the electron gun and in the tube neck. These impurities can originate inside the tube as residue from the faceplate screen or internal coating such as dag, or from outside the tube from assembly line contaminants or weld splashes. The occurrence of arcing results in the removal of the accelerating force from the cathode ray beam producing zero brightness on the display device during the arc event. This eliminates the dynamic load on the high voltage power supply which rises to a very high value resulting in CRT arc cascading. This excessively high voltage transient energy overstresses the CRT, high voltage components, and wiring resulting in potential damage to the CRT and associated television receiver circuitry.
One of the primary modes of CRT high voltage performance degradation caused by arc-over involves the G.sub.3 and G.sub.4 electron beam focus grids of the CRT. Conducting particles on the G.sub.3 focus grid, because of the distorted high electric field it produces, emit electrons by field emission. These electrons are accelerated toward the higher potential G.sub.4 grid upon which they impinge resulting in leakage current. Those accelerated electrons which miss the G.sub.4 focus grid impinge on the grid support rods or tube neck and charge up the surface of impact by causing the emission of secondary electrons. When the potential of the charged surface reaches a sufficiently high value, the charge arcs to regions of lower potential initiating CRT arc-over. In addition, the charging of the neck of the CRT produces an electrostatic field of sufficient magnitude to affect the path of accelerated electrons through the inter-grid space. This produces a shift in the convergence of the picture from the position it would have without stray electron emission and CRT neck charging. If arcing now occurs and the source of stray electron emission changes, the electrostatic potential of the charged CRT neck will change resulting in a shift in video presentation convergence. Thus it can be seen that arc-over involving certain terminal pins, particularly those terminal pins associated with the focus elements of the CRT, require extremely accurate control of the breakdown, or arc-over, voltage to insure proper picture tube performance. The spark gap associated with such terminal contacts must minimize extraneous circuit loading so that the arcing event can be closely and precisely controlled.
Various attempts have been made in the prior art to carefully control the arc-over event in the CRT environment. In general, these attempts have involved the incorporation of individual spark gaps in the CRT socket. These spark gaps are designed to breakdown at a particular potential level and to thereby conduct high voltage transients in the CRT to ground potential level. Basically, the spark gap's discharge voltage increases as the distance between spark gap electrodes increases, and decreases as the inter-electrode distance decreases. By accurately regulating the inter-electrode distance, the prior art has been able to, with varying success, utilize spark gaps having reasonably predictable breakdown voltage characteristics. However, since a change of only approximately 0.001 inch results in a change of about 100 volts in discharge voltage, and since, using generally available production practices, it is difficult to more accurately position spark gap electrodes, the prior art has suffered from performance limitations in attempting to control and regulate CRT arc-over.
One approach to incorporating a high voltage spark gap in a CRT socket is disclosed in U.S. Pat. No. 3,869,633. The invention described therein involves the shearing of elongated radial members across their width to form a gap or opening. As the radial members are sheared, the lower edge of the inner, central portion of the gap assembly is bent downward relative to the adjacent outer portion such that the edge is positioned contiguous to or adjacent to the lower edge of the inner portion. An angled surface is formed during this operation which permits dissipation of arc energy preventing the point of arc ignition from overheating and oxidizing and thereby forming a higher resistance barrier across the gap which affects the ignition, or arc-over, voltage required to establish the next arcing event. In addition, the fabrication of this spark gap configuration is amenable to various abrading processes by which erratic surface protrusions effecting precise control of the arc-over initiating voltage and forming a point of lower impedance to facilitate undesirable corona formation may be easily eliminated. While this approach is capable of precisely controlling electrode surfaces in regulating voltage breakdown of the spark gap, it fails to take into account other more important spark gap characteristics such as the length and configuration of the inter-electrode space.
Another approach to providing arc-over protection in a CRT socket is disclosed in U.S. Pat. No. 3,636,412. Described and claimed therein is a method of forming uniformly controlled spark gaps in which an elliptical barrier wall forming an integral part of the central mounting plate of the socket surrounds a ground ring extension and a high voltage terminal contact extension. The relative position of spark gap electrodes is maintained by a pair of bosses, or studs, which are affixed to the socket's cover plate and positioned in the spark gap during socket fabrication. While this approach provides a simple and easily manufactured socket assembly, it is not intended, nor is it capable of, providing accurate spark gap breakdown voltage control for protecting television receiver components and improving picture tube performance.
Still another approach to over voltage protection in a CRT socket involving the use of spark gaps is disclosed in U.S. Pat. No. 3,716,819 involving the rigid positioning of a first conductive member relative to a second conductive member by encapsulating the conductive members in a plastic material. The rigid incorporation of the conducting members in the encapsulating material to form a plurality of spark gaps ensures uniform conductive member spacing and thus provides a means for maintaining the spark gap breakdown voltage at a precise, predetermined value. This system, however, is limited to handling only the lower voltage components of the CRT. In addition, breakdown voltage regulation is provided by accurate electrode spacing without taking advantage of the inherent operating characteristics of the spark gap cavity. The close electrode spacing and simplified design of this spark gap arrangement result in a limited capability to accommodate the high voltage focusing grid system of the CRT.
The present invention, however, takes into account not only precise spark gap electrode displacement but also the design of the spark gap cavity itself in precisely regulating and controlling the breakdown, or arc-over, voltage of high voltage spark gaps in a CRT socket.